Optical fibers are widely used within telecommunication systems for high-speed information transfer, particularly in the core or backbone network. With the advent of fiber to the premises (FTTP), the conduits will further extend to and into commercial and residential premises. Indeed it is a fundamental part of the push to FTTP in e.g. the UK, that substantially all the network comprises optical fiber, extending from the core network to as many end customers, both commercial and residential, as possible. This involves the installation of millions of kilometers of optical fiber across the network, especially at the local access level which had previously been served by copper. To achieve widespread take-up, the optical fiber installation process needs to be speedy, cost- and effort-efficient.
An optical fiber unit, which could comprise a single optical fiber, or a bound bundle of typically 2 to 24 optical fibers, can be installed using what is known as the “blown fiber” method, described in EP 108590. In this method, a fiber unit is propagated along a hollow tube pre-laid along the desired route, where compressed air is fed, or “blown” into the mouth of the tube. The viscosity of the flow of air travelling along inside the tube, together with the friction on the outer surface of the fiber unit, helps to carry the fiber unit along the length of the tube.
The tubes or conduits typically are made of plastic, each with a typical inner diameter of 2.5 to 6 mm or more, and are usually provided in a bound bundle comprising up to 24 or more tubes, which are held together within a protective outer sheath. The tubes usually comprise a continuous span between convenient access points such as surface boxes, inspection chambers down man holes, or near telephone poles. Each fiber conduit tube can receive at least one fiber unit comprising one or more individual optical fibers. Large numbers of tube conduits—and bundles of conduits—are pre-installed across the access network and the distribution network between the local exchanges and the customer premises, typically to locations which may require a future fiber connection.
When it is decided to populate these pre-installed tubes, the fiber cable or unit is installed in each tubular span by blowing it down the conduit from one end; alternatively the unit could be blown down a concatenation of spans in a single step if circumstances permit. This is repeated for each span, or concatenation of spans, until a continuous fiber path has been laid between the end points of the whole route.
A tubular path can be described between two access points, which may be hundreds of meters apart, by a single length of conduit tube. The conduit path may alternatively comprise a number of lengths of physically separate conduit tubes which are connected together, e.g. in series. Either way, it is crucial to choose the correct conduit path during installation, so that the fiber unit emerges at the desired destination end. During installation however, the operator at one of the installation points would be presented with a large and potentially confusing number of conduit tube openings, each representing a path leading to a destination. The tube openings are usually mapped to their destinations, e.g. by color-coding. If however the tube openings are wrongly mapped, or the records otherwise inaccurate, mistakes can result in attempts to identify the correct conduit path and the correct tube opening leading to the desired destination. This is especially so if the working conditions are poor, e.g. in adverse weather up a telephone pole or down a manhole or in poor lighting.
Where the path comprises a number of tube lengths connectorized together, yet another problem may lie in broken connections between lengths of conduit tubes within the network, so that the fiber unit may get lost within the system during installation and never emerge at the destination. Yet another issue may be the possibility that the fiber unit, during installation, could be impeded by an imperfect connection or a tight bend or some other source of friction in the conduit, and again never emerge at the destination.
For any of these or other reasons, the fiber unit may, during installation, emerge in the wrong place, or in an extreme case, not at all. Add to that some uncertainty about the exact length of the conduit route down which the fiber unit is being installed, so that the operator may not even know in a timely manner when something has gone wrong.
The current response to this problem is to use two operators during installation, one at each end of the installation conduit path. The second operator at the destination end of the tubular path is required because the destination or remote end is often some distance away—up to a kilometer or more—from the head end. The operator at the head end is therefore unable to know the status of the remote end during an installation without a second operator located there reporting the status back to the head end.
During a typical installation session, the first operator situated at the head end of the conduit fits a protective slug or bead on the tip of the fiber unit, then feeds this and compressed air into the mouth of the conduit with fiber installation apparatus—known in the art as a “blowing head” which is described in general in WO88/00713. Meanwhile the second operator locates himself at the desired end point of the fiber installation. The process commences by directing compressed air into the mouth of the head end conduit. If the air is directed into the correct conduit so that the tubular path leads to the desired destination, the remote end operator can eventually sense the arrival of the air with an airflow meter temporarily connected to the end of the conduit, or more simply by feeling the airflow exiting the conduit against his hand if the airflow is sufficiently high. He then communicates this to the head end operator by radio or other means, to confirm to the head end operator that the air is applied to the correct conduit. The head end operator upon receiving the news, then blows the fiber unit into the conduit through to the remote end, whereupon the remote end operator advises his colleague on its arrival. The head end operator then turns off the air supply and the blowing head, and the installation process is complete.
This process is labor-intensive as two operators must work on a single installation. This is in turn drives up the overall cost of optical fiber installation, a problem now especially significant in the FTTP context with the considerable installation volumes involved.
Various methods requiring only a single operator installation of blown fiber have been developed, to obtain a significant saving in manpower and cost requirements. In the simplest method, the length of the conduit route is known, allowing the operator to know that the fiber has (probably) arrived at the remote end when the required length of fiber unit has been played out. This relies on the map record of conduit route being up to date and accurate, and presumes a completely smooth and obstruction-free conduit route. Neither of these can be guaranteed in practice.
Another known practice is to install at the remote end of the conduit a barrier of porous material such as an “airstone” which is constructed of a porous material which allows air through but which will stop further progress of the fiber unit. The airstone is temporarily placed at the mouth of the destination remote end of the tube conduit. When the fiber ceases to travel down the tube, this is an indication that the far end of the fiber may have reached the destination end and has been retained by the airstone barrier. However, lack of further progress is ambiguous as to whether the fiber unit has indeed reached the porous airstone at the destination end, or if instead the fiber unit is caught on an obstruction at some intermediate point along the length of the conduit.
These, together with other methods like those described in WO9103756 or WO/9812588, also describe how fiber arrival can be detected by the single operator at the head end of the installation; the initial step of detecting that the compressed air fed into the head tube end is not addressed in the above techniques.
One method, developed by the applicants of the present case and described in WO2006/103419 does describe a device which permits the detection of the arrival of both air as well as the remote end of the fiber unit at the opening of the remote tube end. The device uses a low-mass wind vane which rotates in the presence of airflow, and which rotation is stopped when the fiber end arrives and gets “caught” in the blades of the wind vane. The change in status of the wind vane can be captured as indicia of air or fiber arrival.